A Plausible Path for Life on Enceladus

byPaul GilsteronMarch 2, 2018

Cassini has shown us that the plumes of Enceladus are laden not just with ammonia and carbon dioxide but also traces of methane. Scientists at the University of Vienna (Austria) are not claiming this finding as evidence for life, but they have produced laboratory work showing that at least one kind of microbe could survive in conditions like those within the moon. Couple this with the presence of molecular hydrogen (H2), also found within the plumes, and the existence of microorganisms deep within Enceladus appears at least plausible. Some of the methane found in the Enceladus plumes may turn out to be produced by methanogens.

The microorganism in question is Methanothermococcus okinawensis, which can be found around sea vents in the Okinawa Trough off Japan. In conditions like these, methanogenic archaea can sustain themselves by the chemical nutrients found around hydrothermal vents, a scenario that could likewise exist beneath the Enceladus ice.

Simon Rittmann, working with colleagues in Austria and Germany, put the microbe into Enceladus-like conditions in a series of laboratory tests, varying the amount of molecular hydrogen. Demonstrating the survival of the archaea involved introducing different values for pressure and pH, assuming abundant carbon dioxide and molecular hydrogen’s store of energy. M. okinawensis demonstrated that it could survive, producing methane as a by-product.

Image: Plumes erupting off the surface of Enceladus, an icy moon. Credit: NASA/JPL/SSI.

At the floor of the Enceladus ocean, temperatures above 0 degrees Celsius are likely to exist in a region abundant in rock and minerals. Enough molecular hydrogen could be produced by reactions involving the mineral olivine to sustain these lifeforms. The process is called serpentinization, involving interactions between seawater and rocks in the moon’s mantle that can also produce methane (CH4) and hydrogen sulfide (H2S). The experimental work shows that serpentinization reactions can support a rate of molecular hydrogen production high enough to sustain this kind of organism. As the paper notes:

When simultaneously applying putative gaseous (Table 4) and liquid inhibitors (Supplementary Table 3) under high-pressure conditions, we reproducibly demonstrated that M. okinawensis was able to perform H2 /CO2 conversion and CH4 production under Enceladus-like conditions.

Thus the microorganism survives under the conditions Rittmann and team introduced into the laboratory, producing methane as it grows, a possible source of the methane found in Cassini observations. At this point in the investigation, we can’t rule out abiotic methane either.

Temperature is an interesting variable, as the paper goes on to show:

The mean temperature in the subsurface ocean of Enceladus might be just above 0 °C except for the areas where hydrothermal activity is assumed to occur. In these hydrothermal settings temperatures higher than 90 °C are supposedly possible, and are therefore the most likely sites for higher biological activity on Enceladus. Although methanogens are found over a wide temperature range on Earth, including temperatures around 0 °C, growth of these organisms at low temperatures is observed to be slow.

With enough molecular hydrogen produced through serpentinization to support methane production, the case for searching for methanogenic biosignatures is clear, an investigation explored briefly in the paper. We may be able to detect lipids and hydrocarbons as well as carbon isotope ratios that flag the presence of living organisms. This laboratory work makes the case for the kind of mission that will be needed to study possible Enceladus-based life in situ to learn whether methanogenic organisms are more than an extrapolation.

“We tried to be as broad as possible with our assumptions,” Rittmann said. There are no direct measurements for what exists beneath Enceladus’s ice crust. “No one will be able to tell if these conditions are really occurring on Enceladus,” he said. “However, we did our best to be as careful as possible.”

Is Tyson soliciting Congress for the funding for this submarine? Or Elon Musk? Tyson would have the clout to do so, if he put as much energy into supporting planetary science as he does with his self-promotion.

At the AGU in December 2017 it was announced that gravitational data from Cassini gathered in its last months indicate that Saturn’s rings are much lighter than previously thought. This means they may be very young. Also at the AGU, Sascha Kempf of University of Colorado announced data from Cassinis CDA that indicated a high rate of dust accretion on the rings which given their high albedo implies an age much younger than Saturn. And not to forget Matija Ćuk’s 2016 simulations indicating instabilities in the moon system.

There are some now researchers including Kempf who think it possible that the moon system has experienced multiple cycles of destruction and recoalescence. There are increasing grounds to believe that Enceladus and brethren are in fact very young. Enceladus may today possess conditions conducive to life, but there are serious grounds to doubt whether sufficient time has elapsed for a native biosphere to have established itself on its own. Possible exospermian options notwithstanding.

It’s interesting what implications for the Fermi equation would mean the discovery of life on a satellite with habitability window likely spanning no more than few tens of MYr. If life was transferred there by lithopansparmia (from Earth, early Mars or even Europa maybe), than life is hardier and more adaptable, capable of sustaining itself in quite hostile and chaotic systems. If it is native, than abiogenesis constant is indeed high. (and then the frozen oceans of KBOs should contain many more preserved native biotas)

But if Enceladus itself formed only recently from debris of other moons, than it’s chemical composition is likely less equilibrated than for 4.5 GYr-old moon, providing easier explanation to the complex plume chemistry.

This is a welcome paper that provides some experimental data on the growth of Archaea under Enceladan conditions. What the authors show is that under certain conditions Archaean prokaryote species can thrive and produce the CH4 from H2 and CO2 that is part of the Archaean eon biosignature that was highlighted in the post Detecting Early Life on Exoplanets.

The authors start by indicating the gases found in the plumes by Cassini. I would note that CO is as prevalent in the plume as CO2 which would be a contra-indicator of a biosignature as this gas is commonly found in comets and therefore potentially a significant component of Enceladus’ mass. A biosignature requires that CO be absent to try to rule out abiogenic methanogenesis. I note that the authors attempt to determine if the rate of H2 and CH4 production is both sufficient to allow enough life to use H2 as a substrate and that the CH4 is sufficient to indicate a biogenic cause.

The authors discuss other key molecules to look for in a future mission to show that life must be present, even if whole cells cannot be captured and even possibly cultured.

My overall sense is that this paper leaves the question of whether Enceladus’ plume composition contains a biosignature as remaining ambiguous. Enceladus may support Archaean prokaryote life based on these experiments, which at least, to my mind, is a positive argument for the possibility of such early life, but not for motile, multicellular life, like those originating in the Ediacaran and Cambrian eras.

What we want to know is whether life on Enceladus, if present, is a de novo genesis or the result of panspermia. Life from Earth could make its way to Enceladus. As missions to the outer planets are so sparse, it would be exciting if a mission could manage a sample return so that plume material could be examined on Earth to try to answer that question. At some point, better propulsion and cheaper craft may make this a reality, possibly in my lifetime.

The panspermian thesis is often proposed without regard for probability calculations. For a seeding of Enceladus from Earth the numbers are decidedly dire.
A paper I read a while back might interest some of you. http://online.liebertpub.com/doi/abs/10.1089/ast.2013.1028
Entitled “Seeding Life on the Moons of the Outer Planets via Lithopanspermia” the paper simulated impact probabilities using very generous assumptions. They concluded that 800 million! objects may have been ejected from Earth of sufficient mass to support life (340kg in this simulation) for up to 10 million years. The simulated total time frame was 3.5Gyr. Over that period 14,000 objects were expected to impact Saturn. Given Enceladus’s orbit and gravity, only a very small fraction of those could be expected to impact on the moon. In fact the fraction is 1 to 14,000.
So even with the optimistic scenario presented in this paper, the simulation produced exactly 1 impact from Earthat Enceladus in 3.5 billion years.

It may be also worth noting that an impactor of this mass would not penetrate the ice crust.

There are plenty of variables and unknown factors at play in such extrapolations, but it should be at least clear that panspermian
fertilisation of Enceladus from Earth is unlikely. The numbers for Martian ejecta were even lower.

The calculations apply to lithopanspermia. Microbes blown away from the top of the atmosphere may be far more prevalent, but have a lower survival rate.

The argument about not penetrating the ice barrier seems more relevant to me. This is a good thing if we want to determine that any life in Enceladus is from a separate genesis. Let’s hope our probes don’t prove to be the eventual contamination source that panspermia failed to achieve.

Hopefully plume samples will be sufficient to detect any life before we try for more exotic probes like submarines.

When ejected beyond the ozone layer unshielded microbes will be killed in short order by solar UVC and soft Xray emissions. Only a select list of spores have proven somewhat hardier. The following is an long and interesting if not comprehensive look at the survivability of microbes in simulated space conditions.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2832349/
Here an extract on B. Subtilis which proved particularly resilient.
“Vacuum-induced mutagenesis indicates that the DNA in spores is one of the critical molecules sensitive to exposure to vacuum. Increasing loss of water due to vacuum exposure leads to partial denaturation of the DNA (64). The consequences are DNA strand breaks, which have been identified in cells of D. radiodurans and Halo-G, now identified as Halorubrum chaoviatoris (159), as well as in spores of B. subtilis, after exposure to space vacuum (55, 56, 158). Spores of the triple mutant repair-deficient strain TKJ 8431 (uvrA10 ssp-1 recA1) of B. subtilis, which are deficient in recombination repair, were the most sensitive specimens under conditions of space vacuum (115). Using space simulation facilities, Moeller et al. (170) showed that NHEJ is a highly efficient repair pathway for DNA DSBs induced in spores of B. subtilis by high vacuum. They hypothesize that NHEJ is a key strategy used during spore germination to repair DSBs caused by ultra-high-vacuum-induced extreme desiccation, as well as by other extreme factors, such as UV and ionizing radiation, encountered during prolonged exposure to the harsh environment of space. These results indicate that forced dehydration of DNA in the microenvironment of the spore core might cause unique damage, with mutagenic and finally lethal consequences. Spore survival ultimately depends on the efficiency of DNA repair after rehydration and germination.”

Even if loose spores were able to reach Enceladus more quickly yhan the 8 million years of the meteoroid scenario AND in a germinable state, rehydration and germination would be hindered by extreme cold. Radiolytic degradation continues on the desiccated spore until such time as it is subdued in some manner. Pretty long odds.

Alex, you seem pretty convinced that Earth or Mars based “panspermian” mechanisms are viable. I have never understood where this conviction comes from. I mean I know there were bacteria found in halite that were suspended for 200+ million years that were said to be alive. But that is not comparable to long duration radiation exposure.
Is it then some conviction in probability? From 10e15 individuals there will always be at least one survivor? This is a fallacy.
I’m not saying the theory must be untenable. I simply have never seen an argument that neutralizes my criticisms.
Without being vague like “we don’t know for sure” or “we could be surprised”, can you offer any firm grounds for your belief? Regards, jw ^_^

In general, I am very skeptical that there is life on these worlds. I think panspermia is possible, but I don’t think panspermia is likely to alter this opinion. Having said that, as a biologist by training, I would be delighted by a discovery of local extra-terrestrial life, especially if its biology is very different from ours. Life constantly surprises us with its ingenuity, so I am cautious about dismissing panspermia. Clarke’s 1st Law comes to mind here. If I had to put skin in the game, I would bet that we will more likely find evidence of life on exoplanets than other worlds in our system. I think it is worth looking for life locally and taking this seriously rather than half-heartedly, but I doubt we will actually find any. But again, Clarke’s 1st law.

In the bigger picture, should life prove either very rare (or undetectable), while extremely disappointing, it does open up the possibility that we, or our descendants, might use directed panspermia to change this situation in our galaxy. This is rather life-centric I know, but life does want to replicate.

Ok. Was kind of hoping for an explicit evidence based defense of Earth to Enceladus or Europa seeding. No matter. I’m surprised at your expressed pessimism for nonterrestrial life in the solar system. I am as hopeful as you that we do discover local off world ecosystems. Maybe even more than you. Because there I can currently conceive of no means to unequivocally prove life on exosolar worlds. Not now and not even in the medium term.

There is plenty of speculation on what could be defined as unequivocal. All of it is far beyond the detection limits of any instruments being planned for the next decades. So barring some breakthrough in detection tech or theory, I think our best hopes are the ocean worlds next door. But that will also likely take decades. Even the Enceladus plume fly through idea is far from guaranteed to gather biomass. But of course we should try. I’ve read only one paper attempting to describe a range of fluid dynamics possibilities within the Enceladan surface fissures. They are pretty vague and preliminary. I wish there were more. Finding biological material on Europa would be a herculean undertaking. Plume content here may contain no oceanic components. Europa Clipper will give us a better sense of the ice dynamics. My guess would be direct access via deep boring will be required to find anything definitive. But let’s cross our fingers that we hit pay dirt sooner rather than later.

Ceres in the asteroid belt show great promise, it has all the elements needed for life and warmth from the Sun. It also has a very low gravity well and even with a small impact it could throw thousands of tonnes on ‘potentially’ viable organism into space. Life could have started here and in relatively short order sent organisms all around the solar system.

There is a good chance that the great caution about contamination of these alien biospheres is unwarranted, because this is only a danger in respect of identifying alien life that uses identical self-replicating molecules to our own (DNA/RNA). Surely this is highly unlikely.

If, after due investigation, we find that Enceladus does not, in fact, have life, we should seed it with life. We should do that with every abiotic body we find that’s capable of supporting life.

Given that we don’t really know that there’s life anywhere else, we should view it as a moral imperative to create backup ecosystems, so that life won’t perish from the universe if we are killed off somehow.

Why??? WHY?? These are essentially egocentric and pseudo religious impulses. Have at it if its your thing. I know that thoughts like these are widespread, nigh universal, but seriously, it would be nice if we could not project ourselves onto the universe as if we have something akin to a divine right or destiny. If we are one of a handful of civilizations to explore the cosmos, that’s interesting. But whether there are 10e25 or 0 civilizations is fundamentally irrelevant. As is the prevalence of life in general. A preference for a universe teeming with life is an aesthetic preference with no other primary internal value. “I want it” Well then “ok”. I’m not criticizing you in particular(your posts are often illuminating) but rather the ubiquitous projections of our singular preferences onto the cosmos with all the self certainty and anthropocentrism that marks our species in its interactions within our own ecosystem.

This is not a call to passivity. I simply hope we will consider more aspects of our psyche that are too often regarded as beyond question. We are not emissaries nor missionaries for the project called biological propagation. To feel that way is no more than to ennoble the instinctual remnants of our simian evolution with a pseudo ethical veneer. We are essentially those simians still. Much of what we abhor and love are age old repetitions of animalian psychology. We can embrace some or all. Or reject some or all. But assertions like yours are a subset of commonplace impulses that are too often clothed in high terms when there is nothing highbrow in them at all. It would be nice if we could at least critically engage with such evolutionary biases. It would illuminate our priorities greatly. Not with the goal of valorising them of course(that is the case now), but rather with the goal of reducing the imperial reflex.

Looking at the world does not make me optimistic that this will happen, but we should open our eyes to the fundamental forces not only in the external world but also within our heads.

A preference for a universe teeming with life is an aesthetic preference with no other primary internal value.

You have made similar arguments before. Call it an aesthetic if you want, but life is the only process that increases complexity and opens up endless possibilities and eventually culture at the human level. Does our culture have no value beyond being an aesthetic choice? Is life in general of no value, just some “carbon infestation”? A living universe that can ultimately have agency seems far more valuable than a dead universe that, like a mechanical clock, winds down to a heat death.

If your objection is life’s expansion from Earth, why the problem if the universe is otherwise dead? It isn’t going to offend anyone else in the universe.

Please read more carefully. I did not say human culture has no value. Nor that I value any arbitrary advanced culture (whether human or not) globally as a representation of an aesthetic. What I said was that most of the wishes and desires often expressed when life in the cosmos is discussed are, to be utterly blunt and grossly reductionist, projections of our own endocrinal impulses.

“life is the only process that increases complexity and opens up endless possibilities” This is not anything more than to claim life is the only anti-entropy machine. Yeah. And? Where is the clear cognitive imperative for biological propagation? There isn’t one. It is a personal aesthetic preference only. Or of some mundane practicality. If you prefer agency, that is a more interesting angle. And I share your preference. But my contention is that our (primarily simian male)imperial nature should not be considered akin to divine law. Unfortunately that is our state today.

This is a subtle theme. Apparently barely palpable for most. It runs too counter to our evolutionary roots to which we remain deeply bonded.

Should nascent indigenous life be eradicated or subjugated by our hand. If you counter, as you already have, that a dead world can be seeded with no harm, there are several issues here also that you fail to see. But the most egregious is how do you propose to determine with absolute certainty the inertness of the world you want to seed?

Ultimately we make a determination which is based on practicalities not absolute certainty. Tough luck then if we are wrong. Oh well.

Anyway, I want us to counter the imperial reflex that is so f-ing prevalent literally everywhere our species treads. This, our worst collective anthropocentric blindness is a primitive relic that we will eventually have to overcome if we aren’t to remain the bain of every ecosystem like we are today.

And if we cannot? The urge to propagate us and… um… us-ness is a deeper impulse than our simian roots. I don’t know that I’d call it an imperialist urge, but I see what you’re driving at. Even so, much of humanity still views ‘me and mine first’ as a valid way to lead our lives. I think what you hope for may have to wait for our successors… if we have any.

Life, as a replicator, creates the “imperative”. If life can migrate to new habitats and proliferate, it will. If natural panspermia can occur, life will naturally propagate throughout the universe. If it requires more intelligent agency, then that will be its means.

I’m guessing that you don’t accept Dawkins’ premises in his “The Selfish Gene”.

Should nascent indigenous life be eradicated or subjugated by our hand….. But the most egregious is how do you propose to determine with absolute certainty the inertness of the world you want to seed?

The short answer is that we cannot be certain. That we have agency and can at least make the attempt to be careful is clearly a step up from unthinking life doing the colonizing by chance.

But even if a world is living with bacteria (and viruses), I am not so certain we should not consider seeding it with multicellular, eukaryotic life. By then we will be able to determine the uniqueness of the extant life and make a decision based on that. Even if unique, we might want to put it in a “zoo” so that the planet can be colonized with the life of our choosing.

This is just sad. And I think I will bug out of further discussions with you on this topic. You positively embrace the imperial attitude with your apparent assumption that we in the future will be the arbiters over life’s direction on other worlds. This simultaneously primitive and god like self entitlement is as widespread as it is gross. If you consider it desirable for us to advance as a species then you might consider adding psychological advancements to the vision.

The argument that we should seek greater self knowledge is a compelling one but, if the rest of universe is devoid of life, why not indulge our aesthetic impulses? The universe is very beautiful and interesting, life or no, to my taste. However I have no objection to anyone who’d enjoy seeing life become more widespread pursuing that goal. Life is also beautiful and interesting to me, and might well become more so as it expands into more novel environments. I basically feel very relaxed about the whole shebang – either way the view is amazing!

“Why??? WHY?? These are essentially egocentric and pseudo religious impulses.”

I am in neither camp, egocentric is about thinking only of oneself without regard for the feelings or desires of others. Nor do I lean towards the pseudo religious camp which believes it is a divine right of humanity to colonise the stars. I do believe we should be ambassadors of life because life is all about expansion and multiplication, and the Universe has no thoughts or feelings on the subject one way or the other.

“…it would be nice if we could not project ourselves onto the universe as if we have something akin to a divine right or destiny.”

The Universe is impartial and indifferent to our failures or successes or even to the emergence life. Life is just a very complex chemical reaction, to me anyway, just because we don’t understand that reaction does not mean it is not happening. It is the impartial and indifference of the Universe that life seems to rebuff, to life the Universe it is just material to allow for its expansion and/or multiplication.

“A preference for a universe teeming with life is an aesthetic preference with no other primary internal value. “I want it” Well then “ok”.

Most human beings even animals of which we are a part seek out other life for food, curiosity or even companionship; it seems to be built into our DNA/RNA. I would rather carry on with that hardwired built in preference than become a static unchanging organism that wishes to mimic inert materials.

“…but rather the ubiquitous projections of our singular preferences onto the cosmos with all the self certainty and anthropocentrism that marks our species in its interactions within our own ecosystem”

“But assertions like yours are a subset of commonplace impulses that are too often clothed in high terms when there is nothing highbrow in them at all. It would be nice if we could at least critically engage with such evolutionary biases.”

There is no anthropocentrism, all ‘life’ has functions or behaviours that make them want to expand and/or multiply or change their environment for their own gain.

“We are not emissaries nor missionaries for the project called biological propagation…”

But there is nothing to say we can’t and why not!

“It would illuminate our priorities greatly. Not with the goal of valorising them of course(that is the case now), but rather with the goal of reducing the imperial reflex.”

There will be no imperial reflex because we will be expanding into a material universe which has no opinion one way or the other; we do not come to displace because inert material has no regard for these human traits.

As a technologically capable species we can make a choice whether to remain in the cradle in the cave and just passively observe the Cosmos. Or we can climb out of the cradle and walk to the cosmic shoreline and then set sail over the vast expanse of space and time to distance worlds to be an active part of the Cosmos.

No other species has got that choice or had it; it would be very egocentric of humanity to waste it, life to be life must propagate.

I am grateful to read your considered responses to my little monologue. Although I wish someone had said,
“Yes, I agree”. When I hear reactions to my “thesis” it often simply feels like i am swimming against an inexorable river of the collective evolutionary strictures which absolutely dominate our psyche. I sadly feel quite alone, and yet unmoved by any counter argument I have thus far seen. I don’t have the time to address all of your objections. Most of them were based on misinterpretations. It is a supremely difficult process to extricate oneself from bias. Your rebuttals by all their sympathy and by all their antipathy have one common aspect. They are heavily embedded in anthropo-imperial (or simply species based)bias. Some embrace or justify parts of it as a lex naturalis with which we should find our peace. But, none have apparently grasped to what degree it clouds our judgement. To draw objective conclusions means more often than not, distancing ourselves from our emotions. I am not advocating a calculus of living. But I am calling for a vastly vastly deeper awareness of our hardwired subjectivity. Our imperial reflex and with it’s toxic effects on the ecosystem(including our social fabric) is undeniable. It was a core element in our civilatoric ascendance. But it is toxic nonetheless. I have written elsewhere on this website about the deeply woven destructive impulses of our psyche and the need for a critical engagement. We are tremendously parochial beings. It may need the shock of a deep discourse with an eti to create the necessary impulse to allow most of us to see for the first time how embedded we are in a world of biases we had never even been aware of.

If Enceladus is only 100 million years old, then life may not have had enough time to emerge. However, it may be long enough for protolife to emerge, which would be just as exciting and possibly more informative than discovering life.

Or maybe life can develop there quicker than we used to think. After all, we now know that life evolved on Earth just over 4 billion years ago, when our planet was roughly 500 million years old and not a very nice place to dwell upon otherwise, at least from our vantage point.

If Enceladus is very young, and if an unambiguous, new genesis can be shown, this would be a very strong argument for the ubiquity of life in the universe, IMO. Rare Earth arguments would be diminished, or at least narrowed to higher life forms that would be affected by Earth’s features.

It is my personal preference, but I find the study of how life can emerge from prebiotic chemistry more compelling than the search for alien life per se. So, for me, a young Enceladus with a complex prebiotic chemistry developing in its own fashion would be at least as wonderful as finding life. This gives me the advantage of being able to get very excited about asteroid missions, and not so let down by finding from Mars missions that point away from any extant habitable environments!

Sodium Carbonate has been detected on the flanks of Ceres’ cryovolcano Ahuna Mons. This means that there is still a reservoir of LIQUID WATER beneath the surface of Ceres in that general area!!! Ceres had a global underground ocean in the past where life may have formed. A deep drilling mission to Ceres would be less expensive than a sample return mission to Enceladus, and the turnaround time would be much less.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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